The invention relates to an AM-FM radio receiver having an FM-demodulator which comprises a network which resonates at the FM intermediate frequency.
Radio receivers of this type are generally known. They are inter alia used for the reception of FM-stereo signals (in accordance with the FCC stereo standard). There are, however, also AM-stereo broadcasts, that is to say transmissions in which stereo signals are transmitted in the medium-wave band.
In accordance with the relevant US-standards such an AM-stereo signal consists of a sum signal (i.e. a signal equal to the sum of the instantaneous values the left and in right channels), which, as is the general practice, is modulated on the carrier frequency, and a difference signal which (together with a 5 Hz stereo-program identification signal) is phase-modulated on the same carrier.
Such an AM-stereo signal can be decoded by employing a phase demodulator in addition to the customary AM-demodulator. The phase demodulator output signal, which corresponds to the difference signal, must be combined in a suitable matrix circuit with the amplitude demodulator output signal, which corresponds to the sum signal, in such manner that the signals for the left and the right channels are produced. Such a receiver requires at least one additional phase demodulator.